join _ [] = [] -- this means that join on the empty list is the empty string
join delim strings = foldl1 (\left right -> left ++ delim ++ right ) strings -- this is join implemented with reduce

-- also this could be written as
join delim strings = foldl1 ((++) . (++ delim)) strings

-- or more perlishly
join delim strings = foldl1 (\left right -> concat [left, delim, right]) strings

-- or with autocurrying fun
join = foldl1 . ((++) .) . flip (++)

##</code><code>##

sub join {
    my ( $delim, @strings ) = @_;
    reduce { $a . $delim . $b } @strings;
}
but in this case the concatenation operator is not used directly as the a curried higher order function